Typically, in bubble jet print heads a plurality of electrically resistive heater elements are deposited on a support substrate that is formed e.g. of metal or silicon and has a heat control coating, e.g. SiO.sub.2. Metal electrodes are formed to selectively apply voltage across the heater elements and a protective coating is provided over the heater elements and electrodes. Printing ink is supplied between and heater elements and orifices of the print head and heater elements are selectively energized to a temperature that converts the adjacent ink to steam rapidly, so that a shock wave causes ejection of ink from the related orifice.
As the development and commercial use of the bubble jet technology progresses, there is increased interest in increasing the resolution of those systems. In this context, system resolution can be thought of as the number of pixel drops printed within a given print region (e.g. line length). One way to increase system resolution is to interlace ink drops, e.g. with multiple passes vis a vis a single array of orifices, or by providing a plurality of scanning orifice arrays. This approach simplifies the print head (s) construction, but requires accurate positioning of the print heads vis a vis the print media to assure good registration of the drops from separate passes.
Another way to increase system resolution is to increase the line density of drop ejector subsystems (i.e. orifices and related heater elements) on a single print head. This can be done with one or a plurality of linear orifice arrays (see e.g. U.S. Pat. No. 4,734,717). Photofabrication techniques enable construction of such high density orifice plate and heater subsystems; however, a present limit to increasing system resolution is presented by the difficulty in making electrical connections between a large number of heating resistors formed on a tiny chip and the electrical address electronics of the printer control system.
To reduce the problem presented by the large number of addressing leads, several systems have been proposed for multiplexing the address of (i.e. the provision of energizing current through) the resistive heater elements of bubble jet print heads. For example, U.S. Pat. No. 4,695,853 discloses a bubble jet chip construction wherein an x-y electrode matrix is constructed with one matrix electrode portion underlying a pattern of resistor/diodes and the other matrix electrode portion overlying the resistor/diode pattern. The x-y electrode portions are separated by an electrically insulative layer except at the resistor/diode components where they form "vertically" disposed, sandwiching terminals. While this resistor/diode construction allows for multiplexing (and thus reduces leads and terminals necessary for address), it has several problems. First, during multiplexing as described in the '853 patent, diode reverse leakage to the upper electrode can cause electrolytic attack that can destroy the electrode traces. Second, it is extremely difficult to equalize forward resistance in a diode while maintaining a fixed resistance value in the heater resistor element.
U.S. Pat. No. 4,791,440 discloses another solution to the problem of providing a high resolution print head with a large number of drop-ejection sites. In this approach, one array of electrical connections to the sites is provided on the top side of the chip substrate and another array of site connections is provided on the bottom side of the substrate. A plurality of holes are provided through the substrate material to couple the top and bottom side lead matrices. To further simplify the electrical lead situation, this approach provides also a multiplex address system wherein a plurality of heater arrays are activated at different phases by an array-select voltage pulse, which, when coupled with a particular heat site data pulse, will provide sufficient heater corrent to eject an ink drop. The '440 patent approach is a difficult one to fabricate, necessitating the forming of multiple holes though the substrate and photofabrication work on both sides of the substrate. In some applications coupling to the printer via the chip bottom surface is not possible. Also, the multiplex system causes unnecessary partial energizations of all heater elements during each active phase of an array. This can cause dissolved gas release from the ink resulting in a bubble which blocks jet activity or ink replenishment.